1. Field of the Invention
This invention relates to a charging device which includes a charging member for charging a member to be charged, such as a photosensitive member or the like, and an image forming apparatus which includes the charging device.
2. Description of the Related Art
FIG. 3 a schematic diagram illustrating an electrophotographic image forming apparatus.
In this image forming apparatus, charging device 10, exposure device 11, developing device 12, transfer device 13, cleaning device 15, charge-removing device 16 and the like are disposed around photosensitive member 1, which serves as an image bearing member.
In an image forming operation, photosensitive member 1 rotates in the direction of arrow R1. After the surface of photosensitive member 1 has been uniformly charged by charging device 10, an electrostatic latent image is formed on the surface of photosensitive member 1 by exposure device 11. The electrostatic latent image is developed by providing toner from developing device 12, whereby a toner image is obtained. The toner image is transferred onto a sheet of transfer material P. Sheets of transfer material P are accommodated within sheet-feeding cassette 17. Each sheet of transfer material P is fed to photosensitive member 1 by a sheet-feeding device (not shown). The toner image on photosensitive member 1 is transferred onto the sheet of transfer material P by transfer device 13. The transferred toner image on the sheet is fixed by fixing device 19, and the sheet is then discharged outside the main body of the image forming apparatus as a final copy.
A detailed description will now be provided of the charging device 10. Charging devices are generally classified into two types, i.e., the contact-charging type and the noncontact-charging type, according to whether or not a charging member contacts a photosensitive member. In contact-charging devices, a charging member (e.g., a conductive roller) directly contacts the surface of a photosensitive member, serving as a member to be charged, and the surface of the photosensitive member is charged to a predetermined potential of a predetermined polarity by applying a voltage (e.g., a DC voltage alone, a superposed voltage of a DC voltage and an AC voltage, or the like) to the charging member. These contact-charging devices have advantages in that, for example, the value of the applied voltage can be lower than in noncontact-charging devices, in which, for example, corona charging is utilized, and that the amount of ozone generated in a charging operation can be reduced. Conventional contact-charging devices are disclosed, for example, in U.S. Pat. No. 5,188,309, and Japanese Patent Laid-open Application (Kokai) No. 1-204081 (1989).
In the above-described conventional contact-charging devices, however, since the photosensitive member directly contacts the charging member for charging it, the charging member is contaminated due to adhesion of remaining toner particles, paper power, and the like, on the photosensitive member, thereby causing insufficient charging for the photosensitive member, and further, causing an inferior image.